Electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is connected to the panelboard via line bus bars and neutral bus bars. The electrical power is delivered from the panelboard to designated branch circuits through line and neutral conductors supplying one or more loads. Typically, various types of protective devices are mounted to the bus bars of the panelboard to protect the branch circuits from hazardous electrical conditions and reduce the risk of injury, damage or fires.
Standard circuit breakers are one type of protective device for protecting the branch circuits from certain hazardous electrical conditions. In particular, standard circuit breakers are designed to trip open and interrupt an electric circuit in response to detecting overloads and short circuits. Overload protection is provided by a thermal element which, when heated by the increased current, will cause the circuit breaker to trip and interrupt the power. This can occur when too many loads draw power from the same branch circuit at the same time, or when a single load draws more power than the branch circuit is designed to carry. Short circuit protection is provided by an electromagnetic element that trips when sensing high current flow.
An arcing fault detector is another type of protective device, designed to protect an electrical distribution system from hazardous electrical arc faults, or "arcing faults," which are one of the major sources of fires in residences. Arcing faults occur when electric current "arcs" or luminously discharges across an insulating medium, usually an ionized gas, between conductors. Some specific causes of arcing faults include: loose or improper connections, frayed or ruptured appliance or extension cords, pinched or pierced insulation of construction wire or extension cords, cracked insulation on wire or cords from age, heat, corrosion or bending stress, overheated or overloaded wires or cords and insulation break down in appliances.
Temperatures at the center of an arc can exceed 5000.degree. F. and ignite the conductor insulation and adjacent combustible materials and/or melt the conductor itself (e.g., copper, having a melting temperature of 1980.degree. F.). At a residential voltage of 120 Vac, an arc may be sustained continuously if it is tracking across a surface that is at least partially conductive. For example, the electrical insulation between conductors, over time, can carbonize thereby forming a conductive path that can sustain an arc which can start a fire. Quite often arcs are not sustained continuously, but rather comprise "sputtering arcs" that strike intermittently, extinguish, and strike again. As they sputter, they may eject small molten particles of conductor metal. Fires can be caused from the molten particles or the heat from the arc.
There are three general types of arcing faults: series arcs, line-to-neutral arcs and line-to-ground arcs. Series arcs are those which occur between two ends of a single conductor. For example, series arcs might result from a frayed conductor in a cord which has been pulled apart, or from a loose connection at a receptacle or in a splice.
Line-to-neutral arcs are short circuits which occur between line and neutral conductors of an electrical distribution system. For example, line-to-neutral arcs might result from a cord whose insulation which has been cut by a staple, or from an object, such as a piece of furniture being placed on it. Line-to-ground arcs are those which occur between a line conductor and a grounded conductor, in a three-wire system. Line-to-ground arcs will not occur in a two-wire appliance or extension cord. It should be noted that there are no "neutral-to-ground arcs" because, although electrical faults may occur between a neutral conductor and ground, the level of current is not high enough to produce an arc.
Generally, standard circuit breakers might detect certain types of arcing faults but can not detect all three types of arcing faults. For example, circuit breakers generally will not detect series arcs, because the current in a series arc is limited by the load it serves and is not usually high enough to trip the thermal or electromagnetic elements associated with standard circuit breakers. Similarly, standard circuit breakers quite often will not detect line-to-neutral arcs because, if the circuit has an impedance which is relatively high, the short-circuit current will be well below the level which would cause the standard circuit breaker to trip. Finally, unless the circuit breaker has a built-in ground fault detection capability, it will not be able to detect line-to-ground arcs.
Standard circuit breakers equipped with a ground-fault detection capability (hereinafter referred to as standard "GFI" circuit breakers) usually include a sensing coil for sensing the differential current between the line and neutral conductors. In the absence of a ground fault, the currents in the line and neutral conductors are equal and opposite and the sensing coil does not detect a differential current. When a ground fault occurs, which might result from current flowing from the line conductor to ground or from the neutral conductor to ground, the sensing coil detects a differential current and will cause the ground fault circuit breaker to trip if the current exceeds a predetermined threshold.
The standard GFI circuit breaker may or may not be able to detect line-to-ground arcs, depending on the trip threshold. In residential applications, for example, the trip threshold of a standard GH breaker is normally very low, on the order of 5-6 mA, which is sufficiently low to detect practically any line-to-ground arcs. In commercial or industrial applications, however, the trip threshold of the standard GFI breaker can be much higher, up to hundreds and thousands of Amps, in which case many line-to-ground faults will fall below the threshold and be undetected. In any case, even though standard GFI circuit breakers might detect some line-to-ground arcs, they generally will not detect series arcs or line-to-neutral arcs for the reasons heretofore stated.
Accordingly, there is a need for an arcing fault protection system which provides protection against arcing faults, preferably series arcs, line-to-neutral arcs and line-to-grounds arcs, at current levels which do not trip standard or standard GFI circuit breakers. Preferably, such system should detect all three of the aforementioned faults with a single sensor coupled to the line conductor. There is also a need for a system and method for testing such detection system, which test system preferably uses a test wire coupled to the same sensor coupled to the line conductor. The present invention is directed to addressing these needs.